I'm actually working on this now, and have Espruino calling into JS for network access. I'll get it working with the ESP8266 first, and will then tweak it for the SIM900.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@CKnight posted up about his project here: http://forum.espruino.com/conversations/264103/
I'm actually working on this now, and have Espruino calling into JS for network access. I'll get it working with the ESP8266 first, and will then tweak it for the SIM900.